TWI597392B - Lithium tantalate single crystal substrate, bonding substrate and method for manufacturing the same, and elastic surface acoustic wave device using the same - Google Patents

Description

Lithium niobate single crystal substrate, bonded substrate thereof, manufacturing method thereof, and elastic surface acoustic wave device using the same

The present invention relates to a lithium niobate single crystal substrate and a bonded substrate obtained by bonding the lithium niobate single crystal substrate to the base substrate, and an elastic surface acoustic wave device using the same.

As a component for frequency adjustment and selection of a mobile phone or the like, a surface acoustic wave (SAW) device that uses an comb-shaped electrode (IDT; Interdigital Transducer) that can provoke an acoustic wave on a piezoelectric substrate is used. .

In the elastic surface acoustic wave device, since the size is small, the insertion loss is small, and the function of not passing the wave is not required. As the material, lithium niobate (LiTaO ₃ ; LT) and lithium niobate (LiNbO ₃ ; LN) are used. Electrical material.

On the other hand, in the communication standard of the fourth-generation mobile phone, the frequency band of the transmission-receiving is narrow and the bandwidth is wide. In the case of such communication specifications, the characteristic change due to the temperature of the material for the elastic surface acoustic wave device is insufficient. Small, the mismatch of the frequency selection domain will occur, and the device's filter and bidirectional function will be problematic. Therefore, it is desired to obtain a material for an elastic surface acoustic wave device having a small variation in temperature characteristics and a wide band width.

In the case of the material for a surface acoustic wave device of this type, in Patent Document 1, when the stoichiometric composition LT which is mainly obtained by a vapor phase method is used for copper for an electrode material, the electrode is destroyed when a high electric power is input to the IDT electrode. Destruction is difficult to occur, so it is preferable. Further, in Patent Document 2, the stoichiometric composition LT obtained by the vapor phase method is also described in detail. Patent Document 3 also describes a specific method of annealing a waveguide formed in a ferroelectric crystal of lithium niobate or lithium niobate.

Further, Patent Document 4 describes a piezoelectric substrate for an elastic surface acoustic wave device which is subjected to Li diffusion treatment on a lithium niobate or lithium niobate single crystal substrate. In Patent Document 5 and Non-Patent Document 1, it is described that the frequency-temperature characteristic can be improved by using LT as a surface acoustic wave element in which the LT composition is made Li-rich in the thickness direction by the gas phase equilibrium method. Therefore, it is preferred. [Prior Art Document] [Patent Literature]

[Patent Document 1] Japanese Patent No. 6 652 644 (B1) [Patent Document 3] JP-A-2003-207671 [Patent Document 4] JP-A-2013-66032 [Patent Document 5] ]WO2013/135886 (A1) [Non-Patent Document]

[Non-Patent Document 1] Bartasyte A. et al., "Reduction of temperature coefficient of frequency in LiTaO ₃ single crystals for surface acoustic wave applications", Applications of Ferroelectrics held jointly with 2012 European Conference on the Applications of Polar Dielectrics and 2012 International Symp Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials (ISAF/ECAPD/PFM), 2012 Intl Symp, 2012, Page(s): 1-3

However, the inventors of the present invention conducted detailed studies on the specific methods described in these publications, and found that excellent results are not always obtained in these methods. In particular, in the production method described in Patent Document 5, since the wafer is processed at a high temperature of about 1300 ° C in the gas phase, that is, a high temperature of about 1300 ° C is produced, the deformation of the wafer is large, and the occurrence rate of the fracture is high. The production efficiency is poor, so the price of the material used as the elastic surface acoustic wave device is high. Furthermore, in the manufacturing method, the vapor pressure of Li ₂ O is low, and the degree of modification of the modified sample is uneven according to the distance from the Li source, and the heterogeneity of the quality becomes a big problem in industrialization. .

Further, in the production method described in Patent Document 5, the Li-modified LT is not subjected to a single polarization treatment after the gas phase equilibrium treatment, and the inventors confirmed this point and newly learned that the single polarization treatment was not performed. The Li-modified LT has a problem that the Q value of the SAW device is small.

Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a small deformation without cracks and flaws, and which has better temperature characteristics than conventional rotary Y-cut LiTaO ₃ substrates, and is electrically and mechanically bonded. A lithium niobate single crystal substrate having a large Q factor and a high Q value of the device, and a bonded substrate bonded to the base substrate and an elastic surface acoustic wave device using the substrate.

The inventors of the present invention conducted intensive studies to achieve the above object, and found that if a substrate having a substantially same composition is subjected to Li diffusion gas phase treatment and modified, it is obtained in the thickness direction of the substrate, which is closer to the substrate surface. When the concentration is higher, the concentration distribution range in which the Li concentration is decreased from the center of the substrate is obtained, so that the modification of the crystal structure having the same Li concentration in the vicinity of the center portion in the thickness direction of the substrate is not obtained. When used for an elastic surface acoustic wave device or the like, the piezoelectric oxide single crystal substrate having small deformation, no cracks and scratches, and excellent temperature characteristics. Furthermore, it was found that the range of Li modification and the presence or absence of a single polarization treatment have an influence on the Q value of the device, and thus the present invention has been obtained.

That is, the lithium niobate single crystal substrate of the present invention diffuses Li from the surface of the spiral Y-cut LiTaO ₃ substrate having a crystal orientation of 35° Y to 49° Y-cut, and has a Li concentration on the substrate surface and the inside of the substrate. LiTaO ₃ single crystal substrate with different concentration distributions, characterized in that the LiTaO ₃ single crystal substrate is subjected to a single polarization treatment, and the acoustic surface acoustic wave transmitted from the surface of the substrate to the surface of the LiTaO ₃ substrate or the acoustic wave of the leaking elastic surface is 5 ~15 times the depth, with a slightly similar Li concentration.

Further, the Li concentration distribution of the present invention is preferably such that the closer the Li concentration is, the closer the Li concentration is to the substrate surface of the S-cut LiTaO ₃ substrate, the more the Li concentration decreases from the center of the substrate, and the Li and Ta on the surface of the substrate are preferable. The ratio is Li:Ta=50-α:50+α, and α is preferably in the range of -0.5<α<0.5. Further, Fe having a concentration of 25 ppm to 150 ppm in the substrate is preferable.

Further, the lithium niobate single crystal substrate of the present invention can be used as a bonded substrate by bonding to a base substrate. In this case, as the bonding substrate, it is preferable to remove the LiTaO ₃ surface layer on the opposite side of the bonding surface, but at least a part of the portion having substantially the same Li concentration is left, and further, Si, SiC, and spinel are used as the base substrate. Any one of the stones is preferred.

The lithium niobate single crystal substrate and the bonded substrate of the present invention are very suitable as materials for the elastic surface acoustic wave device.

According to the present invention, it is possible to provide a lithium niobate single crystal substrate having a temperature characteristic superior to that of the conventional rotary Y-cut LiTaO ₃ substrate, a large electrical mechanical coupling coefficient, and a high Q value of the device. Furthermore, it is also possible to provide an elastic surface acoustic wave device using the single crystal substrate at a low cost, and the device can be suitably applied to a broadband domain band which is necessary in a smart phone.

Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments.

The lithium niobate single crystal substrate of the present invention has a concentration profile in which the Li concentration is different between the surface of the substrate and the inside of the substrate. Further, in the thickness direction of the substrate, the closer the Li concentration is, the higher the concentration of Li in the vicinity of the center portion of the substrate, and the lower the concentration of Li, which is preferable because it is easy to fabricate. A substrate having such a concentration distribution range of Li can be produced by diffusing Li from the surface of the substrate by a known method.

Here, the "concentration distribution" means a change in continuous concentration.

Furthermore, the lithium niobate single crystal substrate of the present invention is characterized in that it has substantially the same Li from the surface of the substrate to the surface acoustic wave transmitted on the surface of the LiTaO ₃ substrate or the depth of the acoustic wave of the leaking elastic surface is 5 to 15 times. concentration. If the range of the Li concentration which is substantially the same is from the surface of the substrate to the surface acoustic wave transmitted on the surface of the LiTaO ₃ substrate or the depth of the acoustic wave of the leakage elastic surface is 5 times or more, compared with the LiTaO ₃ substrate which is not subjected to the Li diffusion treatment. It can display Q values of the same degree or above. On the other hand, if the range of the Li concentration which is substantially the same is 15 times the depth of the wavelength, the diffusion of Li takes time, the production efficiency is deteriorated, and the longer the diffusion of Li takes, the more easily the substrate is deformed and the crack occurs. ,not good.

The Li concentration of the lithium niobate single crystal can be evaluated by measuring the Raman shift peak. Regarding the single crystal of lithium niobate, the relationship between the half value width of the Raman shift peak and the Li concentration (the value of Li/(Li+Ta)) is roughly linear ("2012 IEEE "International Ultrasonics Symposium Proceedings" in the non-patent literature). Page(s): 1252-1255, Applied Physics A 56, 311-315 (1993). Therefore, the composition at any position of the oxide single crystal substrate can be evaluated by the expression indicating the relationship.

The relationship between the half-value width and the Li concentration of the Raman shift peak can be obtained by measuring the Raman half-value width of several samples having a known composition and different Li concentrations, but if Raman is used When the conditions are the same, the relational expression described in the above-mentioned non-patent literature or the like can be used. For example, lithium niobate single crystal can be represented by the following formula (1).

<Formula 1> Li/(Li+Ta)=(53.15-0.5FWHM ₁ )/100 Further, "FWHM1" is a half-value width of a Raman shift peak near 600 cm ^-1 , and details of measurement conditions , can refer to the relevant literature.

"Range roughly from the substrate surface having the same Li concentration" refers to the so-called present invention, having a half-value transfer Raman peak 600cm ^-1 near the surface of the substrate width ± 0.2cm ^-1 or extent of the surface of the substrate Li The value of /(Li+Ta) is in the range of ±0.001.

The lithium niobate single crystal substrate of the present invention is characterized in that a single polarization treatment is performed, but the polarization treatment is preferably performed after the Li diffusion treatment, and if the polarization treatment is performed, the Q value is compared with the case where the polarization treatment is not performed. Can grow bigger.

Further, in the lithium niobate single crystal substrate of the present invention, the ratio of Li to Ta on the surface of the substrate is preferably Li:Ta=50-α:50+α, and α is -0.5<α<0.5. The reason for this is that if the ratio of Li to Ta on the surface of the substrate is within the above range, it is possible to judge the surface of the substrate to have a pseudo stoichiometric composition, and particularly to have excellent temperature characteristics.

The lithium niobate single crystal substrate of the present invention, for example, an oxide single crystal substrate having substantially the same composition can be produced by subjecting a gas phase treatment in which Li diffusion is performed from the surface of the substrate to the inside. In the production of an oxide single crystal substrate having a composition of the same composition, a single crystal ingot may be obtained by a known method such as the Czochralski method, and then it may be subjected to a polishing treatment as needed.

Further, in the lithium niobate single crystal substrate of the present invention, Fe may be doped at a concentration of 25 ppm to 150 ppm. A lithium niobate single crystal substrate doped with Fe at a concentration of 25 ppm to 150 ppm can increase the diffusion rate of Li diffusion by about 20%, and the production efficiency of a Li-diffused lithium niobate wafer is improved, which is preferable. As a doping method, when a single crystal ingot is obtained by the Czochralski method, doping of Fe of a lithium niobate single crystal substrate can be achieved by doping a raw material with a suitable amount of Fe ₂ O ₃ .

Further, the polarization treatment of the embodiment of the present invention can be carried out by a known method, and the vapor phase treatment can be carried out by embedding the substrate in a powder containing Li ₃ TaO ₄ as a main component in the following examples. However, the state of ingredients and substances is not limited to these. Further, the substrate subjected to the vapor phase treatment may be subjected to further processing and processing as needed.

The lithium niobate single crystal substrate of the present invention can be bonded to various base substrates to form a bonded substrate. The base substrate is not particularly limited and may be appropriately selected depending on the purpose, but one of Si, SiC, and spinel is preferably used.

Further, in the case of producing the bonded substrate of the present invention, the LiTaO ₃ surface layer on the opposite side of the bonding surface can be removed, but at least a part of the portion in which the Li concentration of the lithium niobate single crystal substrate is substantially the same is left. Thereby, a bonded substrate having excellent characteristics as an elastic surface acoustic wave element is obtained.

The elastic surface acoustic wave device produced by using the lithium niobate single crystal substrate and the bonded substrate of the present invention is excellent in temperature characteristics, and is particularly suitable for use as a member of a fourth generation mobile phone or the like. [Examples]

Hereinafter, examples and comparative examples of the present invention will be specifically described.

<Example 1> In Example 1, first, a 4-inch diameter lithium niobate single crystal ingot in a ratio of Li:Ta having a substantially identical composition of a single polarization treatment of 48.5:51.5 was cut into 42°. The lithium plated lithium niobate substrate was cut into a thickness of 370 μm. Thereafter, the surface roughness of each of the sliced wafers was adjusted to an arithmetic mean roughness Ra of 0.15 μm and finally to a thickness of 350 μm, as needed.

Then, the inside of the watch was surface-polished and finished into a quasi-mirror substrate having an Ra value of 0.01 μm, and embedded in a powder composed of Li, Ta, and O containing Li ₃ TaO ₄ as a main component. In this case, the powder containing Li ₃ TaO ₄ as a main component is a mixture of Li ₂ CO ₃ :Ta ₂ O ₅ powder at a molar ratio of 7:3, and is calcined at 1300 ° C for 12 hours. . Further, such a powder containing Li ₃ TaO ₄ as a main component is placed in a small container, and a plurality of sliced wafers are embedded in the Li ₃ TaO ₄ powder.

Further, the small container was placed in an electric furnace having an N ₂ atmosphere and heated at 975 ° C for 100 hours to diffuse Li from the surface of the diced wafer toward the center portion. Thereafter, annealing treatment was performed at 800 ° C for 12 hours in the cooling process of the treatment, and thereafter, the wafer was applied with an electric field of 4000 V/m in the direction of the roughly + Z-axis between 770 ° C and 500 ° C of the further cooling process, after which, A process of changing the temperature to room temperature is performed. Further, after the treatment, the rough side was sandblasted to have a Ra value of about 0.15 μm, and the large mirror side was polished to 3 μm to prepare a plurality of lithium niobate single crystal substrates.

One of the lithium niobate single crystal substrates thus produced was separated from the outer peripheral side by a laser Raman spectrometry apparatus (LabRam HR series manufactured by HORIBA Scientific, Ar ion laser, spot size: 1 μm, room temperature). In any portion of 1 cm or more, the half value width of the Raman shift peak near the index of 600 cm ^{-1 of the} amount of diffusion of Li was measured from the surface to the depth direction, and the result of the Raman distribution shown in Fig. 1 was obtained.

From the results of FIG. 1, it is understood that the lithium niobate single crystal substrate has a different Raman half-value width on the surface of the substrate and the inside of the substrate, and a Raman half value in a depth direction of the substrate from 0 μm to about 18 μm. The width is 5.9 to 6.0 cm ^-1 , which is about a certain amount. Further, at a further deep position, the closer to the center portion of the substrate, the larger the value of the Raman half-value width tends to be.

Further, the Raman half-value width at a depth of 80 μm in the thickness direction of the lithium niobate single crystal substrate is 9.3 cm ^-1 , although omitted in the drawing, the Raman half-value width of the center position of the thickness of the substrate Also 9.3cm ^-1 .

As can be seen from the results of FIG. 1 above, in Example 1, the Li concentration in the vicinity of the substrate surface and the inside of the substrate is different, and the closer to the substrate surface, the higher the Li concentration, and the lower the Li concentration in the substrate depth direction. The scope of the distribution. Further, it was confirmed that the Li concentration was approximately the same from the surface of the LiTaO ₃ substrate to a depth of 18 μm.

Furthermore, from the result of FIG. 1, the Raman half-value width is about 5.9 to 6.0 cm ^-1 from the surface of the substrate of the lithium niobate single crystal to the depth of 18 μm in the depth direction, and the formula (1) can be used. The composition in this range is about Li/(Li+Ta)=0.515 to 0.52, so that it can be confirmed as a pseudo stoichiometric composition.

Further, since the Raman half-value width of the central portion in the thickness direction of the substrate of the lithium niobate single crystal is about 9.3 cm ^-1 , the value of Li / (Li + Ta) is also obtained by the above formula (1). Therefore, it can be confirmed as a substantially identical composition.

As described above, in the case of the rotary Y-cut LiTaO ₃ substrate of the first embodiment, the range from the surface of the substrate to the decrease in the concentration of Li or the range in which the concentration is completed is a pseudo-stoichiometric composition, and the central portion in the thickness direction of the substrate is substantially the same. Composition. Further, the position where the Li concentration starts to decrease or the position where the Li concentration ends to be large is a position of 20 μm from the surface of the substrate to the thickness direction.

Then, the deformation of the 4-inch lithium niobate single crystal substrate on which Li diffusion was performed was measured by a laser interference method, and the value was found to be a small value of 60 μm, and no crack or crack was observed.

Further, a small piece cut out from a 4 inch 42° Y-cut lithium niobate single crystal substrate on which Li diffusion was performed was used as a d33/d15 piezoelectric measuring instrument (ZJ-3BN type) manufactured by the Institute of Vocal Music of the Chinese Academy of Sciences. The voltage waveform generated by the vertical vibration applied in the thickness direction of each of the main surfaces and the inside was measured to obtain a piezoelectric reverberation waveform at all places of the wafer. Since the lithium niobate single crystal substrate of the first embodiment has piezoelectricity in the surface of the substrate, it can be used as a single polarization-polarized surface acoustic wave device.

Then, the 42° Y-cut lithium niobate single crystal substrate subjected to the Li diffusion treatment obtained in Example 1 was subjected to a sputtering treatment to form an Al film having a thickness of 0.2 μm, and thereafter, a resist coating was performed in the exposer. The electrode pattern of the ladder filter and the resonator of one stage was exposed and developed, and the electrode of the SAW device was set by RIE (Reactive Ion Etching). Further, one wavelength of the patterned one-stage ladder filter electrode is 2.33 μm in the case of the in-line resonator, and one wavelength in the parallel resonator is 2.47 μm. Further, the resonator unit for evaluation was made to have a 1-wavelength of 2.50 μm.

In the one-stage ladder type filter, the SAW waveform characteristics are confirmed by an RF detector, and the result shown in FIG. 2 can be obtained. In FIG. 2, for the comparison, the same electrode as described above was formed on a 42° Y-cut lithium niobate single crystal substrate on which Li diffusion treatment was not performed, and the measurement results of the SAW waveform were also shown.

From the results of FIG. 2, it was confirmed that in the SAW filter composed of the 42° Y-cut lithium niobate single crystal substrate subjected to the Li diffusion treatment, the insertion loss was 3 dB or less, and the frequency width was 93 MHz. The center frequency of the device is 1745MHz. On the other hand, in the SAW filter composed of a 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment, the insertion loss is 3 dB or less and the frequency width is 80 MHz, and the center frequency of the filter is 1710 MHz. .

Further, the temperature of the base becomes about 16 ° C to 70 ° C, and the anti-resonance frequency corresponding to the frequency on the right side of the depth of FIG. 2 and the temperature of the resonance frequency corresponding to the frequency on the left side of the deep (deep) The coefficients were confirmed separately, and it was found that the temperature coefficient of the resonance frequency was -21 ppm/° C., the temperature coefficient of the anti-resonance frequency was -42 ppm/° C., and the average frequency coefficient of temperature was -31.5 ppm/° C. For comparison, the temperature coefficient of the 42° Y-cut lithium niobate single crystal substrate which was not subjected to Li diffusion treatment was also confirmed, and the temperature coefficient of the resonance frequency was found to be -33 ppm/° C., and the temperature coefficient of the antiresonance frequency was obtained. -43 ppm / ° C, the average frequency temperature coefficient is -38ppm / °C.

Therefore, as a result of the above, it is understood that the lithium niobate single crystal substrate of the first embodiment has a band insertion of 1.2 dB or less in the insertion loss of the filter of the present embodiment as compared with the substrate on which the Li diffusion treatment has not been performed. width. In addition, as for the temperature characteristics, the average frequency coefficient of temperature is about 6.5 ppm/° C. compared with the substrate which is not subjected to the Li diffusion treatment, and the temperature characteristics are small, so that the temperature characteristics are good.

Then, the SAW waveform shown in FIG. 3 was obtained from the 1-port SAW resonator having a wavelength of 2.5 μm produced by the 42° Y-cut lithium niobate single crystal substrate subjected to the Li diffusion treatment of Example 1. Further, in FIG. 3, a similar one-port SAW resonator was produced from a 42° Y-cut lithium niobate single crystal substrate which was not subjected to Li diffusion treatment, and the results of the obtained SAW waveform were collectively shown.

Further, from the result of the SAW waveform of FIG. 3, the electromechanical coupling coefficient k ^{2 is} calculated by the following formula (2) while obtaining the values of the antiresonant frequency and the resonance frequency, as shown in Table 1, and Examples In the 42° Y-cut lithium niobate single crystal substrate subjected to Li diffusion treatment, the electromechanical coupling coefficient k ² is 7.7%, which is a 42° Y-cut lithium niobate single that has not been subjected to Li diffusion treatment. A value of about 1.2 times that of the crystalline substrate.

<Formula 2> K ² calculation formula: Fr : resonance frequency fa : anti-resonance frequency

In addition, in FIG. 4, the SAW resonator of the first embodiment shows the relationship between the real part and the imaginary part of the input impedance (Zin) and the frequency, and the following equation (3) using the BVD model ( The calculated value of the input impedance calculated by "John D. et al., "Modified Butterworth-Van Dyke Circuit for FBAR Resonators and Automated Measurement System", IEEE ULTRASONICS SYMPOSIUM, 2000, pp. 863-868" in the non-patent literature. The relationship between frequencies is also illustrated. Further, from the results of the graph curves A and B of Fig. 4, it can be understood that the values of the input impedance measured by the examples and the calculated values of the BVD model are very consistent.

Further, in Table 1, the result of the Q value obtained by the following formula (3) is shown. In Fig. 5, the measured value of the Q cycle of the SAW resonator and the calculated value of the BVD model are shown together. Further, in the Q cycle, the real part of the input impedance (Zin) is taken as the horizontal axis, and the imaginary part of the input impedance (Zin) is shown as the vertical axis.

From the result of the Q cycle curve C in FIG. 5, it can be confirmed that the value of the input impedance measured in the first embodiment is very consistent with the calculated value of the BVD model, and the Q value obtained by the following formula (3) of the BVD model can be used. Say it is a proper value. Furthermore, regarding the Q cycle, it can be confirmed that the Q value is also large if the radius is large.

In addition, in Table 1 and FIG. 5, for the comparison, the result of the 42° Y-cut lithium niobate single crystal substrate which is not subjected to the Li diffusion treatment (refer to the Q cycle curve D in FIG. 5) is also collectively shown. It can be confirmed that Q of Example 1 is the same as or higher than Q of the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment.

<Formula 3> among them:

<Example 2> In Example 2, first, in the same manner as in Example 1, a lithium niobate single crystal substrate having a substantially similar Li concentration from the surface of the substrate to a depth of 18 μm was produced. Then, the surface of the substrate was polished to 2 μm, and a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 16 μm was used.

Further, the obtained lithium niobate single crystal substrate was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Further, if the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, the depth from the surface of the substrate having the same Li concentration is 6.4 wavelength.

The lithium niobate single crystal substrate of Example 2 has a larger electromechanical coupling coefficient k ² and a higher temperature characteristic than the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment, and the Q value is also the same. The value of the degree or above.

<Example 3> In Example 3, first, a lithium niobate single crystal substrate having a Li concentration which was approximately the same depth from the surface of the substrate to 18 μm was produced in the same manner as in Example 1. Then, the surface of the substrate was polished at 4 μm to obtain a lithium niobate single crystal substrate having a substantially uniform Li concentration from the surface of the substrate to a depth of 14 μm.

Further, the obtained lithium niobate single crystal substrate was evaluated in the same manner as in Example 1. The results are shown in Table 1. Further, when the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, the depth from the surface of the substrate to the range in which the Li concentration is the same is 5.6 wavelength.

In the lithium niobate single crystal substrate of Example 3, the electromechanical coupling coefficient k ^{2 is} large, the temperature characteristics are good, and the Q value is the same as compared with the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment. The value of the degree or above.

<Example 4> In the same manner as in Example 4, first, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was produced in the same manner as in Example 1. Then, the surface of the substrate was subjected to polishing at 5.5 μm, and a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 12.5 μm.

Further, the obtained lithium niobate single crystal substrate was evaluated in the same manner as in Example 1, and as a result, it is shown in Table 1. Further, the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, and the depth from the surface of the substrate to the range of the Li concentration is 5.0 wavelength.

The lithium niobate single crystal substrate of Example 4 has a large electromechanical coupling coefficient k ² and excellent temperature characteristics as compared with a 42° Y-cut lithium niobate single crystal substrate which has not been subjected to Li diffusion treatment, and the Q value is also The same degree or more.

<Example 5> In Example 5, first, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was produced in the same manner as in Example 1. Then, this substrate and a 200 μm-thick Si substrate are used in the non-patent document "Takagi H. et al, "Room-temperature wafer bonding using argon beam activation" From Proceedings-Electrochemical Society (2001), 99-35 (Semiconductor Wafer Bonding: Science) , the method of room temperature bonding described in "Technology, and Applications V), 265-274." is joined to form a bonded substrate. Specifically, the cleaned substrate is mounted in a high vacuum chamber, and the surface of the substrate is irradiated and activated by a high-speed atomic beam of argon gas neutralized by ion beam, and then the lithium niobate single crystal substrate and the Si substrate are bonded.

Further, the joint interface of the bonded substrate was observed by a transmission electron microscope, and as shown in FIG. 6, the pseudo-stoichiometric composition of the bonding interface was interlaced between the atoms of LiTaO ₃ and Si, and the bonding was strong.

In addition, grinding and polishing were performed, and the bonded substrate was allowed to exist in the range of 18 μm from the joint interface to the LiTaO ₃ side, and a bonded substrate in which a Li-spreaded Y-cut LiTaO ₃ substrate and a tantalum substrate were bonded was obtained.

Then, the bonded substrate thus obtained was evaluated in the same manner as in Example 1. The results are shown in Table 2. As a result, it was found that the bonded substrate of Example 5 also exhibited a large electromechanical coupling coefficient and a value Q, and the temperature characteristics were also excellent.

<Example 6> In Example 6, first, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was produced in the same manner as in Example 1. Then, the substrate was bonded to a 200 μm thick Si substrate by a room temperature bonding method described in the non-patent document to obtain a bonded substrate.

Further, the joint interface of the bonded substrate was observed by a transmission electron microscope, and it was found that, in the same manner as in Example 5, the pseudo-stoichiometric composition of the joint interface of LiTaO ₃ and Si was intertwined to form a strong bond.

In addition, grinding and polishing were carried out so as to remain in the range of 1.2 μm from the joint interface to the LiTaO ₃ side, thereby obtaining a bonded substrate of a Li-spreaded Y-cut LiTaO ₃ substrate and a tantalum substrate.

Then, the bonded substrate thus obtained was evaluated in the same manner as in Example 1. The results are shown in Table 2. From this result, it was found that the bonded substrate of Example 6 also had a large electromechanical coupling coefficient and a Q value, and the temperature characteristics were also excellent.

[Comparative Example] In the comparative examples shown below, a lithium niobate single crystal substrate was produced in the same manner as in Example 1 except that the single polarization treatment was not performed. <Comparative Example 1> In Comparative Example 1, in the temperature lowering process after the Li diffusion treatment, the electric field in the +Z-axis direction was not applied between 770 ° C and 500 ° C (the single polarization treatment was not performed), and A lithium niobate single crystal substrate was produced in the same manner as in Example 1.

The lithium niobate single crystal substrate of Comparative Example 1 showed the same Raman distribution as that of Example 1, and had approximately the same Li concentration from the surface of the substrate to a depth of 18 μm.

Further, a small piece cut out from a 4 inch 42° Y-cut lithium niobate single crystal substrate on which Li diffusion of Comparative Example 1 was carried out was used as a d33/d15 piezoelectric measuring instrument (ZJ- by the Institute of Vocal Music, Chinese Academy of Sciences). 3BN type), it is observed whether the vertical vibration in the thickness direction is applied to each of the main surface and the inside to arouse the voltage waveform, and the piezoelectric response is not obtained in all places of the wafer. Therefore, it was confirmed that the lithium niobate single crystal substrate of Comparative Example 1 had no piezoelectricity in all thickness directions in the surface of the substrate, and there was no single polarization.

On the other hand, the small piece was placed in the d15 unit, vibration was applied to the horizontal direction of the substrate, and a piezoelectric response was obtained in the thickness direction. Although the lithium niobate single crystal substrate of Comparative Example 1 did not have a thickness direction in the thickness direction vibration. The piezoelectric response, but if vibration is applied in the horizontal direction of the substrate, piezoelectricity is generated, and it can be confirmed as a special piezoelectric body.

The lithium niobate single crystal substrate of Comparative Example 1 was evaluated in the same manner as in Example 1. The results are shown in Table 1. From this result, the lithium niobate single crystal substrate of Comparative Example 1 has a large electromechanical coupling coefficient k ² and excellent temperature characteristics as compared with the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment. , its Q value is small.

<Comparative Example 2> In Comparative Example 2, first, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was produced in the same manner as in Example 1. Then, the surface of the substrate was polished to 8 μm to prepare a lithium niobate single crystal substrate having a Li concentration which was approximately the same from the surface of the substrate to a depth of 10 μm.

Further, the lithium niobate single crystal substrate of Comparative Example 2 was evaluated in the same manner as in Example 1. The results are shown in Table 1. Further, when the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, it is found that the depth in the range from the same Li concentration on the substrate surface is 4.0 wavelength.

From this result, the lithium niobate single crystal substrate of Comparative Example 2 has a large electromechanical coupling coefficient k ² and excellent temperature characteristics as compared with the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment. As indicated by the Q cycle curve E in Fig. 5, its Q value is small.

<Comparative Example 3> In Comparative Example 3, first, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was produced in the same manner as in Example 1. Then, the surface of the substrate was subjected to 12 μm polishing to obtain a lithium niobate single crystal substrate having a substantially uniform Li concentration from the surface of the substrate to a depth of 8 μm.

Further, the lithium niobate single crystal substrate of Comparative Example 3 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Further, the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, and a depth of 3.2 in the range in which the Li concentration is the same from the surface of the substrate is obtained.

From the results, it was found that the lithium niobate single crystal substrate of Comparative Example 3 has a large electromechanical coupling coefficient k ² and excellent temperature characteristics as compared with the 42° Y-cut lithium niobate single crystal substrate which is not subjected to Li diffusion treatment. Its Q value is small.

<Comparative Example 4> In Comparative Example 4, first, in the same manner as in Example 1, a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 18 μm was obtained. Then, the surface of the substrate was subjected to 14 μm polishing to obtain a lithium niobate single crystal substrate having approximately the same Li concentration from the surface of the substrate to a depth of 6 μm.

Further, the lithium niobate single crystal substrate of Comparative Example 4 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Further, when the wavelength of the leaky elastic surface acoustic wave transmitted in the X direction of the wafer is normalized, it is found that the depth in the range from the same Li concentration on the substrate surface is 2.4 wavelength.

From this result, the lithium niobate single crystal substrate of Comparative Example 3 was found to have a large electromechanical coupling coefficient k ² and excellent temperature characteristics as compared with the 42° Y-cut lithium niobate single crystal substrate which was not subjected to Li diffusion treatment. However, as indicated by the Q cycle curve F in Fig. 5, the Q value is small.

Table 1

Table 2

A‧‧‧Im (Zin) measured value in Figure 4 and curve calculated by BVD model (solid line and dotted line)
B‧‧‧Re (Zin) measured values in Figure 4 and curves calculated using the BVD model (solid and dotted)
C‧‧‧Q cycle curve of the input impedance (Zin) of Example 1 and the calculated value of the BVD model (dotted line) in Figure 5
D‧‧‧Q cycle curve of the measured value (Zin) of the input impedance (solid line) and the calculated value (dotted line) of the BVD model in the case where there is no Li diffusion processing in Fig. 5
E‧‧‧Measurement value (Zin) of Comparative Example 2 (when the depth of the same Li concentration is 10 μm from the surface of the substrate) and the calculated value of the BVD model (dotted line) Q cycle curve
F‧‧‧Comparative Example 4 in Figure 5 (when the depth of the same Li concentration is 6 μm from the surface of the substrate), the measured value of the input impedance (Zin) (solid line) and the Q of the BVD model calculated value (dotted line) Cycle curve

Fig. 1 is a view showing a Raman distribution of Example 1.

Fig. 2 is a waveform diagram showing insertion loss of the SAW filter of the first embodiment.

Fig. 3 is a waveform diagram showing a SAW resonator of the first embodiment.

4 is a view showing a SAW resonator waveform, an input impedance (Zin) solid line, a dotted line waveform, and a calculated value obtained by the BVD model of the first embodiment.

5 is a graph showing the measured values of the input impedance (Zin) and the BVD model of the SAW resonators of the first embodiment and the comparative examples 2 and 4, and the real part is the horizontal axis and the imaginary part is the vertical axis. Q cycle diagram.

6 is a photograph showing a transmission electron microscope in the vicinity of a bonding interface between LiTaO ₃ and Si of the bonded substrate of Example 5. FIG.

Claims (11)

A lithium niobate single crystal substrate which is diffused Li from the surface of a rotating Y-cut LiTaO ₃ substrate cut from a crystal orientation of 36° Y to 49° Y, and has a concentration of Li on the substrate surface and the inside of the substrate. a distributed LiTaO ₃ single crystal substrate characterized in that the LiTaO ₃ single crystal substrate is subjected to a single polarization treatment, from the surface of the substrate to an elastic surface acoustic wave or a leaky elastic surface transmitted on the surface of the LiTaO ₃ substrate The depth of the sound wave is 5 to 15 times the depth, and has a substantially the same Li concentration. The lithium niobate single crystal substrate according to claim 1, wherein the Li concentration distribution is closer to a surface of the substrate of the rotating Y-cut LiTaO ₃ substrate, and the closer the Li concentration is, the closer to the center portion of the substrate The Li concentration is less. The lithium niobate single crystal substrate according to claim 1 or 2, wherein the ratio of Li to Ta on the surface of the substrate is Li:Ta=50-α:50+α, and α is -0.5<α <0.5 range. The lithium niobate single crystal substrate described in the first or second aspect of the patent application is doped with Fe at a concentration of 25 ppm to 150 ppm in the substrate. A bonded substrate comprising the lithium niobate single crystal substrate according to any one of the above-mentioned items of the first to fourth aspects, which is bonded to a base substrate. In the bonded substrate according to claim 5, the LiTaO ₃ surface layer on the opposite side of the joint surface is removed, but at least a portion of the portion having substantially the same Li concentration remains. The bonded substrate according to claim 5, wherein the base substrate is any one of Si, SiC, and spinel. A flexible surface acoustic wave device characterized in that: The lithium niobate single crystal substrate according to any one of the items 1 to 4, or the bonded substrate according to any one of the items 5 to 7. A method for producing a bonded substrate, comprising: a LiTaO ₃ single crystal substrate having a substrate having a different Li concentration in a substrate; and a LiTaO ₃ single crystal substrate and a base substrate having substantially the same Li concentration from at least one substrate surface to an arbitrary depth Bonding removes the LiTaO ₃ surface layer on the opposite side of the joint surface, but at least a portion of the portion having substantially the same Li concentration is retained. A method for producing a bonded substrate, comprising: a LiTaO ₃ single crystal substrate having a concentration distribution different from a Li concentration in the substrate surface and a substrate, and a LiTaO ₃ single crystal substrate having substantially the same Li concentration from at least one substrate surface to an arbitrary depth The substrate was joined, and the LiTaO ₃ surface layer on the opposite side of the joint surface was removed, and only the portion having substantially the same Li concentration remained. The method for producing a bonded substrate according to claim 9 or 10, wherein the portion in which the Li concentration is substantially the same is a pseudo stoichiometric composition.